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I want to replace underscores with slashes when they are before a four digit number, for example

res_234_2010-xxx -> res_234/2010-xxx
res_12345_1999-yyy -> res_12345/1999-yyy

(In fact, the four digit number should be a valid year, startin with 19xx or 20xx)

Is there some way to achieve it with .replace method, or shall I do it programmatically?

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  • Use a lookahead. regular-expressions.info/lookaround.html. Please show what you've tried, we can't help you fix your code if you don't first make an attempt. Commented Apr 12, 2015 at 2:33

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You can use the following regular expression:

var result = str.replace(/_((?:19|20)\d{2})\b/g, '/$1');

Regular Expression Explanation

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2 Comments

Great answer, I would give you extra points for adding the regex explanation
@opensas, using lookahead. var result = str.replace(/_(?=(?:19|20)\d{2}\b)/g, '/');

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